I've made a little app for Android phones and tablets that you might find useful.

It's called Telescope Calc and it's an easy to use calculator that will output the following parameters for given telescope-eyepiece combination:

resolving power of the telescope

star magnitude limit

diameter of exit pupil

magnification

field of view

surface brightness

You can also save your configurations for later. Additionally, there are 4 predefined formulas for finding minimum, maximum, optimum and wide-field magnifications. You can set the size of your eye pupil in settings, and it will be taken into account when calculating some of those.

It's also optimized for tablets, meaning it makes use of extra space to make itself even more comfortable to use.

It's available now for free from Google Play and SlideME (for those without access to Google Play)